Interstellar pickup ions: Not just theory anymore

Although the far‐flung Pioneer and Voyager spacecraft will not leave the heliosphere (the region of particles and fields emitted by the Sun) for at least another few quadrennia, an important component of the surrounding interstellar medium is already under study in the form of interstellar pickup ions. In the last four years, there have been great advances in the study of these particles, primarily motivated by the new observations from instruments on the Ulysses spacecraft. This article will review the exciting work on interstellar pickup ions which took place during the last quadrennium, concentrating on the efforts of the U.S. scientists and their collaborators. As the Sun moves through interstellar space, the heliospheric magnetic field prevents the ionized portion of the surrounding interstellar gas from entering the solar system, but the population of interstellar neutral atoms flows unimpeded through the essentially collisionless plasma. When these atoms travel close enough to the Sun, solar radiation (in the form of both photons and solar wind particles) acts to ionize them and the new ions must suddenly respond to the electromagnetic fields of the supersonic solar wind. In the reference frame moving with the solar wind, this response consists of an immediate gyration of the new ions about the magnetic field, followed by a rapid isotropization by ambient or self‐generated low‐frequency fluctuations in the plasma. This isotropization in the solar wind frame means that the ions are now convecting with the solar wind ‐ they have been “picked up”. Since the inflowing interstellar atoms move very slowly with respect to the Sun (20 – 25 km/s), they have supersonic speeds in the solar wind frame, and the isotropization of the ions in this frame results in a distinct energetic population of ions with origins in the interstellar medium.